Method and apparatus for multi-spray RRC process with dynamic control

ABSTRACT

A multi-spray RRC process with dynamic control to improve final yield and further reduce resist cost is disclosed. In one embodiment, a method, includes: dispensing a first layer of solvent on a semiconductor substrate while spinning at a first speed for a first time period; dispensing the solvent on the semiconductor substrate while spinning at a second speed for a second time period so as to transform the first layer to a second layer of the solvent; dispensing the solvent on the semiconductor substrate while spinning at a third speed for a third time period so as to transform the second layer to a third layer of the solvent; dispensing the solvent on the semiconductor substrate while spinning at a fourth speed for a fourth time period so as to transform the third layer to a fourth layer of the solvent; and dispensing a first layer of photoresist on the fourth layer of the solvent while spinning at a fifth speed for a fifth period of time.

BACKGROUND

During semiconductor fabrication, a variety of photolithographicprocesses are performed to apply layers to, or perform implants in, asemiconductor wafer. A photosensitive photoresist is applied to thewafer, and patterned using a photomask to form a hard mask for asubsequent deposition or etching process. The cost of photoresist is asignificant material cost in semiconductor fabrication. RRC (ReducingResist Consumption) process is widely used in the Semiconductor industryto decrease the cost of photo resist per wafer. However, the process isoften accompanied with various coating defects (e.g., microbubbles) thatmake it difficult to improve final yield and further reduce resist costper wafer. This disclosure exploit a multi-spray RRC process withdynamic control to improve final yield and further reduce resist cost.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that various features are not necessarily drawn to scale. In fact,the dimensions and geometries of the various features may be arbitrarilyincreased or reduced for clarity of illustration.

FIG. 1 illustrates an exemplary block diagram of a spin coater systemfor coating a semiconductor wafer with a thin film, in accordance withsome embodiments of present disclosure.

FIGS. 2A-2F illustrate cross-sections of a substrate during aspin-coating process, in accordance with some embodiments of presentdisclosure.

FIG. 3 illustrates a flow chart of a method to spin coat a substrate, inaccordance with some embodiments of present disclosure.

FIG. 4 illustrates maps of breakdown voltages measured on a plurality ofMetal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) fabricatedon a substrate using three different spin-coating methods, in accordancewith some embodiments of the present disclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following disclosure describes various exemplary embodiments forimplementing different features of the subject matter. Specific examplesof components and arrangements are described below to simplify thepresent disclosure. These are, of course, merely examples and are notintended to be limiting. For example, it will be understood that when anelement is referred to as being “connected to” or “coupled to” anotherelement, it may be directly connected to or coupled to the otherelement, or one or more intervening elements may be present.

This disclosure presents various embodiments of a multi-spray RRCprocess with dynamic control to improve final yield and further reduceresist cost.

FIG. 1 illustrates an exemplary block diagram of a spin coater system100 for coating a semiconductor wafer with a thin film, in accordancewith some embodiments of present disclosure. It is noted that the system100 is merely an example, and is not intended to limit the presentdisclosure. Accordingly, it is understood that additional functionalblocks may be provided in or coupled to the system 100 of FIG. 1 , andthat some other functional blocks may only be briefly described herein.

In the illustrated embodiment, the spin coater system 100 is to deposita uniform thin film to a surface of a flat substrate 102 usingcentrifugal force. In some embodiments, the thin film can comprisessol-gel precursors and photoresist. In some embodiments, the system 100comprises a chuck 104 for securing the substrate 102 firmly withoutdeflection while operating at a very high rotational speed. In someembodiments, the chuck 104 has a mass that allows for instantaneousdirection and speed change with precise acceleration and decelerationcontrol. In some embodiments, the chuck 104 is a vacuum chuck. In someembodiments, the vacuum chuck 104 comprises a low-profile, O-ring sealfor high-performance vacuum seal. In some other embodiments, the chuck104 comprises an edge-grip chuck for substrates that are sensitive tovacuum contact. In some embodiments, the chuck 104 is attached to amotor (not shown) which is configured to provide precise speed control.

In some embodiments, the system 100 comprises a holder 106 with at leastone nozzle 108/110 for dispensing a coating 112 onto the substrate 102.In the illustrated embodiment, the system 100 comprises 2 nozzles108/110 with a first nozzle 108 for dispensing an reducing resistconsumption (RRC) solvent from a RRC source 114 and a second nozzle 110for dispensing a polymer (e.g., photoresist) from a photoresist source116. In some embodiments, the RRC solvent source 114 and the photoresistsource 116 each comprises a respective pump (not shown) for injectingthe materials into the respective nozzles. In some embodiments, the RRCsolvent and photoresist are both directed through a single nozzle andinjected by a single pump.

In some embodiments, the pump attached to the material source 114/116 isfurther coupled to a controller 120 to control the time and rate of thedispensing of the coating. In some embodiments, the controller 120 isfurther coupled to the motor coupled to the chuck 104 so as to controlthe speed, acceleration/deceleration, and spinning time of the chuck104. In some embodiments, the dispensing of the coating and the spinningof the chuck are synchronized and automatically controlled by thecontroller 120.

In some embodiments, the controller 120 is a representative device andmay comprise a processor, a memory, an input/output interface, acommunications interface, and a system bus. The processor may compriseany processing circuitry operative to control the operations andperformance of the controller 120. In various aspects, the processor maybe implemented as a general purpose processor, a chip multiprocessor(CMP), a dedicated processor, an embedded processor, a digital signalprocessor (DSP), a network processor, an input/output (I/O) processor, amedia access control (MAC) processor, a radio baseband processor, aco-processor, a microprocessor such as a complex instruction setcomputer (CISC) microprocessor, a reduced instruction set computing(RISC) microprocessor, and/or a very long instruction word (VLIW)microprocessor, or other processing device. The processor also may beimplemented by a controller, a microcontroller, an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA), aprogrammable logic device (PLD), and so forth.

In various aspects, the processor may be arranged to run an operatingsystem (OS) and various applications. Examples of an OS comprise, forexample, operating systems generally known under the trade name of AppleOS, Microsoft Windows OS, Android OS, and any other proprietary or opensource OS.

In some embodiments, at least one non-transitory computer-readablestorage medium is provided having computer-executable instructionsembodied thereon, wherein, when executed by at least one processor, thecomputer-executable instructions cause the at least one processor toperform embodiments of the methods described herein. Thiscomputer-readable storage medium can be embodied in the memory.

In some embodiments, the memory may comprise any machine-readable orcomputer-readable media capable of storing data, including bothvolatile/non-volatile memory and removable/non-removable memory. Thememory may comprise at least one non-volatile memory unit. Thenon-volatile memory unit is capable of storing one or more softwareprograms. The software programs may contain, for example, applications,user data, device data, and/or configuration data, or combinationstherefore, to name only a few. The software programs may containinstructions executable by the various components of the controller 120of the system 100.

For example, memory may comprise read-only memory (ROM), random-accessmemory (RAM), dynamic RAM (DRAM), Double-Data-Rate DRAM (DDR-RAM),synchronous DRAM (SDRAM), static RAM (SRAM), programmable ROM (PROM),erasable programmable ROM (EPROM), electrically erasable programmableROM (EEPROM), flash memory (e.g., NOR or NAND flash memory), contentaddressable memory (CAM), polymer memory (e.g., ferroelectric polymermemory), phase-change memory (e.g., ovonic memory), ferroelectricmemory, silicon-oxide-nitride-oxide-silicon (SONOS) memory, disk memory(e.g., floppy disk, hard drive, optical disk, magnetic disk), or card(e.g., magnetic card, optical card), or any other type of media suitablefor storing information.

In one embodiment, the memory may contain an instruction set, in theform of a file for executing a method of generating one or more timinglibraries as described herein. The instruction set may be stored in anyacceptable form of machine-readable instructions, including source codeor various appropriate programming languages. Some examples ofprogramming languages that may be used to store the instruction setcomprise, but are not limited to: Java, C, C++, C#, Python, Objective-C,Visual Basic, or .NET programming. In some embodiments a compiler orinterpreter is comprised to convert the instruction set into machineexecutable code for execution by the processor.

In some embodiments, the I/O interface may comprise any suitablemechanism or component to at least enable a user to provide input (i.e.,configuration parameters, etc.) to the controller 120 and the controller120 to provide output control to the other components of the system 100(e.g., pump, motor, etc.).

In some embodiments, the substrate 102 includes a silicon substrate.Alternatively, the substrate 102 may include other elementarysemiconductor material such as, for example, germanium. The substrate102 may also include a compound semiconductor such as silicon carbide,gallium arsenic, indium arsenide, and indium phosphide. The substrate102 may include an alloy semiconductor such as silicon germanium,silicon germanium carbide, gallium arsenic phosphide, and gallium indiumphosphide. In one embodiment, the substrate 102 includes an epitaxiallayer. For example, the substrate 102 may have an epitaxial layeroverlying a bulk semiconductor. Furthermore, the substrate 102 mayinclude a semiconductor-on-insulator (SOI) structure. For example, thesubstrate 102 may include a buried oxide (BOX) layer formed by a processsuch as separation by implanted oxygen (SIMOX) or other suitabletechnique, such as wafer bonding and grinding. In some embodiments, thesubstrate 102 comprises a glass substrate, and a flexible filmsubstrate.

In some embodiments, the RRC solvent is an organic solvent widely usedto reduce photoresist consumption. In some embodiments, the RRC solventcomprises at least one of the following: Propylene Glycol Methyl Ether(PGME), propylene glycol methyl ether acetate (PGMEA), Cyclohexanone,Cyclopropanone, Methyl-N-Pentyl Ketone, Ethyl_Lactate,n-methyl-2-pyrrolidone (NMP) and a combination thereof. In someembodiments, the photoresist is sensitive to light comprising organicresin and solvent. In some embodiments, the photoresist comprisesSEPR-432. It should be noted that these are just examples and are notintend to be limiting.

FIGS. 2A-2F illustrate cross-sections 200 of a substrate 102 during aspin-coating process, in accordance with some embodiments of presentdisclosure. In the illustrated embodiment, a RRC solvent is dispersedthrough a first nozzle 108 to a substrate 102, wherein the substrate 102is supported on a chuck 104. In some embodiments, the chuck 104 iscoupled to a motor that can provide precise control of the direction andspeed of the rotational motion of the substrate 102. In someembodiments, the RRC solvent is dispersed while the substrate isspinning at a first speed R1 for a first time period. In someembodiments, R1 is in a range of 1500-3500 Revolution per minute (RPM).During this step, the RRC solvent forms a first layer 202 on thesubstrate which covers an area at the center of the substrate with aradius of L1 and a thickness of t1, as shown in FIG. 2A.

After T1, the substrate 102 changes its speed to a second speed R2 andthe RRC solvent continues to be dispensed for a second period of timeT2. In some embodiments, R2 is in a range of 500-2500 RPM and R2 is lessthan R1. During this step, the first layer 202 of the RRC solventtransforms to a second layer 204 covering an area in the center of thesubstrate 102 with a radius of L2 and a thickness of t2, wherein t2 isgreater than t1 and L2 is substantially comparable to L1.

After T2, the substrate 102 changes its speed to a third speed R3 andthe RRC solvent continues to be dispensed for a third period of time T3.In some embodiments, R3 is in a range of 2500-5000 RPM and R3 is greaterthan R1. During this step, the second layer 204 of the RRC solventtransforms to a third layer 206 covering an area in the center of thesubstrate 102 with a radius of L3 and a thickness of t3, wherein t3 isless than t2 and L3 is comparable to the radius of the substrate 102.

After T3, the substrate 102 changes its speed to a fourth speed R4 andthe RRC solvent continues to be dispensed for a fourth period of timeT4. In some embodiments, R4 is in a range of 100-2000 RPM and R4 is lessthan R2. During this step, the third layer 206 of the RRC solventtransforms to a fourth layer 208 covering the entire surface of thesubstrate 102 with a radius L4 and a thickness t4, wherein t4 is greaterthan t3. In some embodiments, a total time for spinning the RRC solvent,T_(RRC), is a summation of T1, T2, T3 and T4. In some embodiments, T1and T3 each takes 30-50% of T_(RRC); and T2 and T4 each takes 20-30% ofT_(RRC). In some embodiments, TRRC is determined by the type of the RRCsolvent, which is in a range of 0.6-12 seconds.

After T4, the substrate 102 changes its speed to a fifth speed R5 for afifth period of time T5. In the illustrated embodiment, the fourth layer208 of the RRC solvent transforms to a fifth layer 210 of the RRCsolvent covering the entire surface of the substrate 102 with a radiusL5 and a thickness t5. In some embodiments, the thickness t5 is smallerthan the thickness t4.

After the formation of the fifth layer 210 of the RRC solvent, thephotoresist is dispensed on the surface of the substrate 102 through asecond nozzle 110, wherein the second nozzle 110 is first moved to thecenter of the stage while the first nozzle 208 moved out of the centerof the stage. In some embodiments, during this step, the substrate 102spins at the fifth speed R6 for the fifth period of time T6, wherein R6and T6 are determined by the type of photoresist and feature size to bemanufactured on the substrate 102. For example, T6=2 second for SEPR-432and R6 is in a range of 50-5000 RPM. During this step, a first layer 212of the photoresist is formed on top of the fifth layer 210 of the RRCsolvent on the substrate 102. In some embodiments, the thickness of thefirst layer 212 of the photoresist is controlled by the type of thephotoresist, R6 and T6.

FIG. 3 illustrates a flow chart of a method 300 to spin coat a substrate102, in accordance with some embodiments of present disclosure. In someembodiments, the operations of method 300 are performed by therespective components illustrated in FIGS. 1-2 . For purposes ofdiscussion, the following embodiment of the method 300 will be describedin conjunction with FIGS. 1-2 . The illustrated embodiment of the method300 is merely an example for generating a masking map. Therefore, itshould be understood that any of a variety of operations may be omitted,re-sequenced, and/or added while remaining within the scope of thepresent disclosure.

The method 300 starts with operation 302 in which a first layer 202 ofan RRC solvent is dispensed on a substrate 102 according to someembodiments. In some embodiments, the first layer 202 of the RRC solventis formed while the substrate 102 spins at a first speed R1 for a firsttime period T1. In some embodiments, R1 is in a range of 1500-3500Revolution per minute (RPM). In some embodiments, the first layer 202has a first thickness t1 and covers an area at the center of thesubstrate 102 with a radius of L1. In some embodiments, the RRC solventis dispensed on the surface of the substrate 102 through a first nozzle108.

The method 300 continues with operation 304 in which the first layer 202is transformed to a second layer 204 of the RRC solvent according tosome embodiments. In some embodiments, the second layer 204 of the RRCsolvent is formed while the substrate 102 spins at a second speed R2 andthe RRC solvent continues to be dispensed on the substrate 102 for asecond period of time T2. In some embodiments, R2 is in a range of500-2500 RPM and R2 is less than R1. In some embodiments, the secondlayer 204 covers an area in the center of the substrate 102 with aradius of L2 and a thickness of t2, wherein t2 is greater than t1 and L2is substantially comparable to L1.

The method 300 continues with operation 306 in which the second layer204 is transformed to a third layer 206 of the RRC solvent according tosome embodiments. In some embodiments, the third layer 206 is formedwhile the substrate 102 spins at a third speed R3 and the RRC solventcontinues to be dispensed for a third period of time T3. In someembodiments, R3 is in a range of 2500-5000 RPM and R3 is greater thanR1. In some embodiments, the third layer 206 covers an area in thecenter of the substrate 102 with a radius of L3 and a thickness of t3,wherein t3 is less than t2 and L3 is comparable to the radius of thesubstrate 102.

In some embodiments, after the operation 304 and prior to the operation306, at least one step of dispensing of the solvent while spinning thesemiconductor wafer can be performed. In some embodiments, thesemiconductor wafer can be spun at a speed which is different from thesecond speed and the third speed.

The method 300 continues with operation 308 in which the third layer 206is transformed to a fourth layer 208 of the RRC solvent according tosome embodiments. In some embodiments, the fourth layer 208 is formedwhile the substrate 102 spins at a fourth speed R4 and the RRC solventcontinues to be dispensed for a fourth period of time T4. In someembodiments, R4 is in a range of 100-2000 RPM and R4 is less than R2. Insome embodiments, the fourth layer 208 covers the entire surface of thesubstrate 102 with a radius L4 and a thickness t4, wherein t4 is greaterthan t3.

In some embodiments, a total time for spinning the RRC solvent, T_(RRC),is a summation of T1, T2, T3 and T4. In some embodiments, T1 and T3 eachtakes 30-50% of T_(RRC); and T2 and T4 each takes 20-30% of T_(RRC). Insome embodiments, TRRC is determined by the type of the RRC solvent,which is in a range of 0.6-12 seconds.

The method 300 continues with operation 310 in which a first layer 210of a photoresist is formed on the substrate 102 according to someembodiments. In some embodiments, the first layer 210 of the photoresistis formed while the substrate 102 spins at a fifth speed R5 and thephotoresist is dispensed for a fifth period of time T5. In someembodiments, the photoresist is dispensed on the surface of thesubstrate 102 through a second nozzle 110. In some embodiments, R5 andT5 are determined by the type of photoresist and feature size to bemanufactured on the substrate 102. For example, T5=2 second for SEPR-432and R5 is in a range of 50-5000 RPM. In some embodiments, the thicknessof the first layer 210 of the photoresist is controlled by the type ofthe photoresist, R5 and T5.

FIG. 4 illustrates maps 400 of breakdown voltages measured on aplurality of Metal-Oxide-Semiconductor Field Effect Transistors(MOSFETs) fabricated on a substrate 102 using three differentspin-coating methods, in accordance with some embodiments of the presentdisclosure. In the illustrated embodiment, the breakdown voltages aremeasured by applying a first probe on a gate terminal of a MOSFET and asecond probe on a body terminal of the MOSFET. In some embodiments, afirst voltage of V₀ (e.g., −1.5 V) is applied between the first probeand the second probe, a current between a source terminal and a drainterminal of the MOSFET is measured. In some embodiments, a secondvoltage of V_(stress), a current is measured after a fixed interval(e.g., a few milliseconds) when the voltage between the first and secondprobes returns to V₀ from +(V₀+V_(step)). In some embodiments,V_(stress)=V_(0+n)×V_(step), wherein n is a positive integer. Thecurrent is then compared with a predefined threshold current value. Insome embodiments, the predefined threshold current value is 1microampere. When the current is greater than the predefined thresholdcurrent value, the V_(stress) is a breakdown voltage of the MOSFET. Inthe illustrated embodiments, a plurality of chips with a size of 23×9millimeter are fabricated on a wafer. Each of the plurality of chips hasan identical layout design and comprises a plurality of MOSFET devices.The breakdown voltages of the same MOSFET devices at the same positionof the plurality of chips are measured and plotted to determine adistribution of defects introduced in three different photoresistcoating processes. Plot 402 shows the distribution of defects on a waferusing a 1-time spin coating of the RRC solvent; plot 404 shows thedistribution of defects on a wafer using a 4-time spin coating of theRRC solvent; and plot 406 shows the distribution of defects on a waferusing method 300. In the illustrated embodiments, a chip with a forwardslash has a breakdown voltage in a range of −9.583 and −34.717 V, whichare considered as “no defects”; and a chip with a backward slash has abreakdown voltage in a range of greater than −9.587 V or smaller than−34.717 V. The distribution and number of defects in chips fabricatedusing the method 300 are improved compared to that fabricated using the1-time spin-coating of the RRC solvent and comparable to that fabricatedusing the 4-time spin coating of the RRC solvent. The multi-spray RRCprocess with dynamic control improves final yield and further reducesresist cost.

In one embodiment, a method, includes: dispensing a first layer ofsolvent on a semiconductor substrate while spinning at a first speed fora first time period; dispensing the solvent on the semiconductorsubstrate while spinning at a second speed for a second time period soas to transform the first layer to a second layer of the solvent;dispensing the solvent on the semiconductor substrate while spinning ata third speed for a third time period so as to transform the secondlayer to a third layer of the solvent; dispensing the solvent on thesemiconductor substrate while spinning at a fourth speed for a fourthtime period so as to transform the third layer to a fourth layer of thesolvent; and dispensing a first layer of photoresist on the fourth layerof the solvent while spinning at a fifth speed for a fifth period oftime.

In another embodiment, a method, includes: dispensing a first layer ofsolvent on a semiconductor substrate while spinning at a first speed fora first time period; dispensing the solvent on the semiconductorsubstrate while spinning at a second speed for a second time period soas to transform the first layer to a second layer of the solvent;dispensing the solvent on the semiconductor substrate while spinning soas to transform the second layer to a third layer of the solvent;dispensing the solvent on the semiconductor substrate while spinning ata third speed for a third time period so as to transform the third layerto a fourth layer of the solvent; dispensing the solvent on thesemiconductor substrate while spinning at a fourth speed for a fourthtime period so as to transform the fourth layer to a fifth layer of thesolvent; and dispensing a first layer of photoresist on the fifth layerof the solvent while spinning at a fifth speed for a fifth period oftime.

Yet, in another embodiment, a method, includes: dispensing a first layerof solvent on a semiconductor substrate while spinning at a first speedfor a first time period; dispensing the solvent on the semiconductorsubstrate while spinning at a second speed for a second time period soas to transform the first layer to a second layer of the solvent;dispensing the solvent on the semiconductor substrate while spinning ata third speed for a third time period so as to transform the secondlayer to a third layer of the solvent; dispensing the solvent on thesemiconductor substrate while spinning at a fourth speed for a fourthtime period so as to transform the third layer to a fourth layer of thesolvent; and dispensing a first layer of photoresist on the fourth layerof the solvent while spinning at a fifth speed for a fifth period oftime, wherein the third speed is greater than the first speed, whereinthe first speed is greater than the second speed, and wherein the secondspeed is greater than the fourth speed, wherein the first time periodand the third time period each equals to 30-50% of a summation of thefirst time period, the second time period, the third time period, andthe fourth time period, and wherein the second time period and thefourth time period each equals to 20-30% of a summation of the firsttime period, the second time period, the third time period, and thefourth time period.

The foregoing outlines features of several embodiments so that thoseordinary skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodimentsintroduced herein. Those skilled in the art should also realize thatsuch equivalent constructions do not depart from the spirit and scope ofthe present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A method, comprising: dispensing a first layer ofsolvent on a semiconductor substrate while spinning at a first speed fora first time period such that the first layer of the solvent covers afirst area at a center of the semiconductor substrate with a firstradius and a first thickness; dispensing the solvent on thesemiconductor substrate while spinning at a second speed for a secondtime period so as to transform the first layer to a second layer of thesolvent such that the second layer of the solvent covers a second areaat the center of the semiconductor substrate with a second radius and asecond thickness; dispensing the solvent on the semiconductor substratewhile spinning at a third speed for a third time period so as totransform the second layer to a third layer of the solvent such that thethird layer of the solvent covers a third area at the center of thesemiconductor substrate with a third radius and a third thickness;dispensing the solvent on the semiconductor substrate while spinning ata fourth speed for a fourth time period so as to transform the thirdlayer to a fourth layer of the solvent such that the fourth layer of thesolvent covers a fourth area at the center of the semiconductorsubstrate with a fourth radius and a fourth thickness, wherein the firstand second radii are different from the third and fourth radii, and thefirst and third thicknesses are different from the second and fourththicknesses; and dispensing a first layer of photoresist on the fourthlayer of the solvent while spinning at a fifth speed for a fifth periodof time.
 2. The method of claim 1, wherein the first speed is in a rangeof 1500-3500 revolution per minute (RPM).
 3. The method of claim 1,wherein the second speed is in a range of 500-2500 RPM.
 4. The method ofclaim 1, wherein the third speed is in a range of 2500-5000 RPM.
 5. Themethod of claim 1, wherein the fourth speed is in a range of 100-2000RPM.
 6. The method of claim 1, wherein the third speed is greater thanthe first speed, wherein the first speed is greater than the secondspeed, and wherein the second speed is greater than the fourth speed. 7.The method of claim 1, wherein the first time period and the third timeperiod each equals to 30-50% of a summation of the first time period,the second time period, the third time period, and the fourth timeperiod, wherein the summation is in a range of 0.6-12 second.
 8. Themethod of claim 1, wherein the second time period and the fourth timeperiod each equals to 20-30% of a summation of the first time period,the second time period, the third time period, and the fourth timeperiod, wherein the summation is in a range of 0.6-12 second.
 9. Themethod of claim 1, wherein the dispensing the solvent is performed at aflow rate of 60-90 milliliter per minute (mL/m).
 10. A method,comprising: dispensing a first layer of solvent on a semiconductorsubstrate while spinning at a first speed for a first time period suchthat the first layer of the solvent covers a first area at a center ofthe semiconductor substrate with a first radius and a first thickness;dispensing the solvent on the semiconductor substrate while spinning ata second speed for a second time period so as to transform the firstlayer to a second layer of the solvent such that the second layer of thesolvent covers a second area at the center of the semiconductorsubstrate with a second radius and a second thickness; dispensing thesolvent on the semiconductor substrate while spinning so as to transformthe second layer to a third layer of the solvent such that the thirdlayer of the solvent covers a third area at the center of thesemiconductor substrate with a third radius and a third thickness;dispensing the solvent on the semiconductor substrate while spinning ata third speed for a third time period so as to transform the third layerto a fourth layer of the solvent such that the fourth layer of thesolvent covers a fourth area at the center of the semiconductorsubstrate with a fourth radius and a fourth thickness, wherein the firstand second radii are different from the third and fourth radii, and thefirst and third thicknesses are different from the second and fourththicknesses; dispensing the solvent on the semiconductor substrate whilespinning at a fourth speed for a fourth time period so as to transformthe fourth layer to a fifth layer of the solvent; dispensing a firstlayer of photoresist on the fifth layer of the solvent while spinning ata fifth speed for a fifth period of time.
 11. The method of claim 10,wherein the first speed is in a range of 1500-3500 revolution per minute(RPM).
 12. The method of claim 10, wherein the second speed is in arange of 500-2500 RPM.
 13. The method of claim 10, wherein the thirdspeed is in a range of 2500-5000 RPM.
 14. The method of claim 10,wherein the fourth speed is in a range of 100-2000 RPM.
 15. The methodof claim 10, wherein the third speed is greater than the first speed,wherein the first speed is greater than the second speed, and whereinthe second speed is greater than the fourth speed.
 16. The method ofclaim 10, wherein the first time period and the third time period eachequals to 30-50% of a summation of the first time period, the secondtime period, the third time period, and the fourth time period, whereinthe summation is in a range of 0.6-12 second.
 17. The method of claim10, wherein the second time period and the fourth time period eachequals to 20-30% of a summation of the first time period, the secondtime period, the third time period, and the fourth time period, whereinthe summation is in a range of 0.6-12 second.
 18. A method, comprising:dispensing a first layer of solvent on a semiconductor substrate whilespinning at a first speed for a first time period such that the firstlayer of the solvent covers a first area at a center of thesemiconductor substrate with a first radius and a first thickness;dispensing the solvent on the semiconductor substrate while spinning ata second speed for a second time period so as to transform the firstlayer to a second layer of the solvent such that the second layer of thesolvent covers a second area at the center of the semiconductorsubstrate with a second radius and a second thickness; dispensing thesolvent on the semiconductor substrate while spinning at a third speedfor a third time period so as to transform the second layer to a thirdlayer of the solvent such that the third layer of the solvent covers athird area at the center of the semiconductor substrate with a thirdradius and a third thickness; dispensing the solvent on thesemiconductor substrate while spinning at a fourth speed for a fourthtime period so as to transform the third layer to a fourth layer of thesolvent such that the fourth layer of the solvent covers a fourth areaat the center of the semiconductor substrate with a fourth radius and afourth thickness, wherein the first and second radii are different fromthe third and fourth radii, and the first and third thicknesses aredifferent from the second and fourth thicknesses; and dispensing a firstlayer of photoresist on the fourth layer of the solvent while spinningat a fifth speed for a fifth period of time, wherein the third speed isgreater than the first speed, wherein the first speed is greater thanthe second speed, and wherein the second speed is greater than thefourth speed, wherein the first time period and the third time periodeach equals to 30-50% of a summation of the first time period, thesecond time period, the third time period, and the fourth time period,and wherein the second time period and the fourth time period eachequals to 20-30% of a summation of the first time period, the secondtime period, the third time period, and the fourth time period.
 19. Themethod of claim 18, wherein the first speed is in a range of 1500-3500revolution per minute (RPM), the second speed is in a range of 500-2500RPM, the third speed is in a range of 2500-5000 RPM, and the fourthspeed is in a range of 100-2000 RPM.
 20. The method of claim 18, whereinthe summation is in a range of 0.6-12 second.